1. Field of the Invention
The present invention relates to patch antennas applicable mobile or wireless communications systems and more particularly, to a single-mode patch antenna for circularly polarized waves and a dual-mode patch antenna operable as a linearly polarized antenna at a frequency and a circularly polarized antenna at another frequency, which are capable of easy optimization in both impedance matching and axial ratio adjustment.
2. Description of the Prior Art
In the field of mobile communication, various types of patch antennas have been extensively used because of their advantage of compactness, which are equipped with a plate-shaped dielectric substrate and a conductor patch formed on the surface of the substrate.
FIG. 1 shows a prior art circularly polarized patch antenna, which is shown based on the paper written by P. C. Sharma et al., "Analysis and Optimized Design of Single Feed Circularly Polarized Microstrip Antenna", IEEE TAP 1983, Vol., AP-31, No. 6, pp. 949-955.
In FIG. 1, a rectangular conductor patch 151 serving as a radiating element is formed on the surface of a rectangular dielectric substrate 150. The two long sides of the patch 151 have a length of L1 and two short sides thereof have L2. A plate-shaped grounding conductor 153 serving as a ground plane is formed on the opposite surface of the substrate 150 to the patch 151. The reference numeral 152 denotes a feedpoint through which electric power is fed to the patch 151.
The above-described prior-art patch antenna of FIG. 1 has the following problem.
Specifically, with the prior-art patch antenna shown in FIG. 1, electric power is supplied to the patch 151 through the single feedpoint 152 and the antenna structure is very simple. Therefore, there is a problem that a degree of freedom to optimize both the axial ratio setting for circularly polarized waves and the impedance matching at a specific frequency is insufficient.
Also, a monopole or dipole may be additionally provided as an additional radiating element above the patch 151 in order to add another antenna function. In this case, to form a way of feeding electric power to the monopole or dipole thus added, a square or rectangular aperture needs to be formed in the patch 151 to expose the underlying surface of the dielectric substrate 150. However, the aperture causes another problem that the location adjustment of the feedpoint 152 becomes more difficult. Also, it causes a further problem that unwanted shift of the axial ratio of elliptically polarized waves is generated and this shift cannot be fully compensated by simply changing the location of the feedpoint 152.
FIG. 2 shows another prior-art circularly polarized patch antenna usable at two different frequencies of f1 and f2, which is a dual-frequency antenna. This is shown based on the paper written by D. Sanchez-Hernandez et al., "Single-fed dual band circularly polarized microstrip patch antennas", 26th EuMc 9-12 Sep. 1996, Prague, pp. 273-277.
In FIG. 2, a rectangular patch 257 serving as a radiating element is formed on the surface of a rectangular dielectric substrate 255. The shape and size of the patch 257 are designed to have a resonant frequency at f1. A plate-shaped grounding conductor 253 serving as a ground plane is formed on the opposite surface of the substrate 255 to the patch 257. The reference numeral 256 denotes a feedpoint through which electric power is fed to the patch 257.
Unlike the prior-art antenna shown in FIG. 1, the patch 257 has a L-shaped slit 258A formed near its long side 257a and a L-shaped slit 258B formed near its short side 257b. The slit 258A extends inwardly by a specific length from a point on the long side 257a and then, bends at a right angle and runs parallel to the side 257a by a specific length. The part of the slit 258A which is parallel to the long side 257a is longer than that which is perpendicular thereto.
Similarly, the slit 258B extends inwardly by a specific length from a point on the long side 257b and then, bends at a right angle and runs parallel to the side 257b by a specific length. The part of the slit 258B which is parallel to the long side 257b is longer than that which is perpendicular thereto.
Here, it is supposed that the parts of the slits 258A and 258B, which are respectively in parallel to the long and short sides 257a and 257b, have a same width of SLSL and a same length of LSL. If the values of the width WLSL and the length LSL are suitably adjusted, a filter effect is generated due to the existence of the slits 258A and 258B, resulting in a resonant frequency at f2 which is different from f1. Thus, the prior-art patch antenna shown in FIG. 2 have two resonant frequencies at f1 and f2, which means that it serves as a double-frequency antenna.
With the prior art patch antenna shown in FIG. 2, however, if a square or rectangular aperture is formed in the patch 257a in order to add a monopole or dipole over the patch 257 as an additional radiating element, the difficulty in location adjustment of the feedpoint 256 is increased due to existence of the slots 258A and 258B. Moreover, since the axial ratio adjustment and the impedance matching become more difficult than the prior art patch antenna shown in FIG. 1, the addition of a monopole or dipole above the patch 257 is extremely difficult to be realized.
To increase the ease in the axial ratio adjustment and impedance matching at different frequencies, several methods have been developed and proposed. However, all the proposed methods require to provide two arrays of patches on a same dielectric substrate. As a result, a large space of patches is necessary and the size of an antenna is increased, which is contrary to the advantage of compactness of patch antennas.
Furthermore, the difficulty in the axial ratio adjustment and the impedance matching is increased by the L-shaped slots 258A and 258B, because the addition of the slots 258A and 258B generates some deviation in the axial ratio and/or the matched impedance.
Actual communications systems require low-cost, small-sized circularly polarized antennas having a well-adjusted axial ratio and a well-matched impedance. However, as far as the inventors know, the prior-art antennas including the above-described antennas shown in FIGS. 1 and 2 provide only one of a well-adjusted axial ratio and a well-matched impedance. This means that the prior-art antennas essentially requires a compromise between the axial ratio adjustment and the impedance matching.
On th other hand, in recent years, there have been the growing need for dual-mode patch antennas capable of operation as a linearly polarized antenna at a frequency and a circularly polarized antenna at another frequency. This need is one the basis of the intention to cope with several different communication systems, such as the ground wave communication systems using linearly polarized waves and the satellite communication systems using circularly polarized waves.
As explained previously, the prior-art patch antenna shown in FIG. 2 is operable at the two different frequencies f1 and f2. However, this antenna is dedicated to circularly polarized waves. Therefore, if it is applied to linearly polarized waves, it will produce a lot of cross polarization components. Thus, it is unable to be operated as a dual-mode patch antenna.
FIG. 3 shows a prior-art dual-mode patch antenna, which is equipped with two patches designed respectively for circularly and linearly polarized waves. This antenna is shown based on the same paper written by P. C. Sharma et al. as that cited with reference to FIG. 1.
As shown in FIG. 3, a first rectangular patch 362 and a second parallelogrammic patch 363 are formed on the surface of a rectangular dielectric substrate 361. These two patches 362 and 363 are apart from each other at a short distance. A plate-shaped grounding conductor 364 serving as a ground plane is formed on the opposite surface of the substrate 361 to the patches 362 and 363. The reference number 365 denotes a feedpoint through which electric power is supplied to the first patch 362. The reference numeral 366 denotes a microstrip line formed on the surface of the substrate 361 to be connected to the second patch 363 at its short side. The line 366 is designed for supplying electric power to the second patch 363.
The first patch 362, which is used for circularly polarized waves, has a shape of a parallelogram with two long sides 362a and 362c of SL1 and two short sides 362b and 362d of SL3. By setting precisely the location of the feedpoint 365 on the patch 362, the impedance matching and the axial ratio setting can be suitably established at a desired frequency (i.e., a first frequency).
The second patch 363, which is used for linearly polarized waves, has a shape of a rectangle with two long sides (i.e., resonant sides) 363aand 363c of RL and two short sides (i.e., non-resonant sides) 363b and 363d of RW perpendicular to the long sides. The resonance length of the patch 363 is set to be equal to a half wavelength at another desired frequency (i.e., a second frequency).
The first and second patches 362 and 363 and the microstrip line 366 are formed on the dielectric sheet 361 by a well-known printing process or the like.
With the prior-art dual-mode patch antenna shown in FIG. 3, the first parallelogrammic patch 362 dedicated to circularly polarized waves and the second rectangular patch 363 dedicated to linearly polarized waves are provided on the same dielectric substrate 361. Therefore, to prevent the electromagnetic coupling between the patches 362 and 363, these patches 362 and 363 need to be located apart from each other at a specific distance or longer. As a result, this antenna has a problem that it occupies a larger space than that having a single patch and that it raises the fabrication cost.
Moreover, the use of the two patches 362 and 363 may cause another problem that the volume required by the two patches 362 and 363 generates a difficulty in mechanical support of the antenna. It may cause a further problem that two feed systems are necessary to supply electric power to the patches 362 and 363, resulting in a high antenna profile.